1. Field of the Invention
The present invention pertains to towed-array marine seismic surveys, and, more particularly, to towed-array marine seismic surveys employing generally advancing curved sail lines, also known as a “coil shoot”.
2. Description of the Related Art
This section of this document is intended to introduce various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of related art. That such art is related in no way implies that it is also prior art. The related art may or may not be prior art. It should therefore be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
The exercise of examining subterranean geological formations for hydrocarbon deposits is known as “seismic surveying”. Sometimes the geological formations lie beneath a body of water. This type of seismic survey is known as a “marine” seismic survey. Marine seismic surveys may be performed in salt, fresh, or brackish waters and are not limited to saltwater environments.
One type of marine seismic survey is called a “towed-array” seismic survey. In such a survey, a survey vessel tows an array of equipment along a straight sail line. The array usually includes a number of seismic streamers, typically up to eight, that can be several kilometers long. The streamers are populated with a number of instruments, most notably seismic receivers such as hydrophones. Sometimes the array will also include a plurality of seismic sources. As the array is towed along the straight sail line, the streamers ideally straighten and roughly parallel each other.
Sometimes environmental or survey conditions adversely impact the shape of the array. Arrays therefore typically include a number of steering devices. Steering devices are typically included on various parts of the array and, in particular, the streamers. Streamer steering devices such as the WesternGeco Q-FIN™ operate with a global system controller that has an overall view of the streamer shape to determine the steering device instructions. Without such a global controller to coordinate the efforts of each individual steering device, the system would be unable to achieve any meaningful steering objectives.
Steering objectives that have been achieved by the WesternGeco global controller include a fan mode, a constant separation mode, and a demanded feather mode. In the fan mode, the streamers are spread relative to each other to avoid collision and tangling. In the constant separation mode, streamer separation is equal from front to tail of the streamer spread, overcoming the so-called trouser effect that results from vessel wake and causes uneven streamer separations towards the spread center. In the demanded feather mode, the streamers have a specified “feather”. This mode is especially useful for so-called 4D surveys, but it is also useful for coverage with respect to the feather along the adjacent line.
The global controller for the spread steering elements in general, and the streamer steering devices in particular, that implement these modes use knowledge of the relative position of those steering elements they must coordinate with to obtain the geophysical or operational safety objective the mode is targeting. The controller possesses communication and processing capabilities to make the necessary steering decisions in a timely manner.
The controller's role places some constraints on its location. For example, a local controller located in the steering device must have a global or at least a subset of the global coordinates of the steering devices it is coordinating with available to it in a time period that allows a steering action that is a reaction to recent positions. A more general solution is to locate a global controller on the towing vessel and design the system to deliver steering instructions based on the global two-dimensional or three-dimensional coordinates of the relevant points in the spread.
Today, a new type of towed-array marine seismic survey is being developed known as a “coil shoot”. In a coil shoot, the array is towed on a generally advancing, curved path rather than a straight line. When shooting a coil survey, the streamers are changing their orientation with respect to any local ocean or sea current as they traverse through the coil or curved path. This causes the streamers to drift away from the desired positions if one of the conventional steering modes is in use as these modes are not designed to maintain a curve shape. Instead the streamers may attempt to keep constant separation or feather angle no matter what the streamer shape.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.